1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically, this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a transversely flowing stream of fluidized catalyst particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in a contacting zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the contacting zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the solids can dramatically affect the performance of the contacting zone. Ideally, the feed is instantaneously dispersed as it enters the riser over the entire cross-section of a stream of solids that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross-section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of particles. However, the dispersing of the feed throughout the particles takes some time so that there is some non-uniform contact between the feed and particles as previously described. Non-uniform contacting of the feed and the particles, for the time it is in the axial contact zone, exposes portions of the feed to the particles for longer periods of time which in turn can produce overcracking and reduce the quality of reaction products.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solid. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. It is believed that as droplet sizes become small enough, they can completely vaporize before contacting the solids. However, it is also known that the improvements in dispersal and vaporization must be balanced against the smaller momentum and decrease in penetration across the catalyst stream that results as feed droplets become smaller. A variety of methods are known for shearing such liquid streams into fine droplets.
U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for a fluid catalytic cracking unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles. U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles. The injection devices of the '540 and '049 patents rely on relatively high fluid velocities and pressure drops to achieve atomization of the oil into fine droplets. Providing this higher pressure drop burdens the design and increases the cost of equipment such as pumps and exchangers that are typically used to supply liquid and gas to the feed injection device. The need to replace such equipment may greatly increase the cost of retrofitting an existing liquid-solid contacting installation with such an injection apparatus.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Catalyst particles first enter the riser with zero or negative velocity in the ultimate direction of riser flow. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser, the turbulence can cause mal-distribution and variation in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser, it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and backmixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 issued to Hammershaimb and Lomas.
There are additional references which show the use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 issued to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom of a vertically confined conduit, and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C.sub.4 minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil.
The orientation of feed injection has also received attention. U.S. Pat. No. 5,139,748 issued to Lomas et al., shows the use of radially directed feed injection nozzles to introduce feed into an FCC riser. The nozzles are arranged in a circumferential band about the riser and inject feed toward the center of the riser. The nozzle arrangement and geometry of the riser maintain a substantially open riser cross-section over the feed injection and downstream riser sections. Feed atomization, lift gas, and radial injection of feed have been used to more uniformly disperse feed over the cross-section of a riser reaction zone. Nevertheless, as feed contacts the hot catalyst, cracking and volumetric expansion of the hydrocarbons causes an increase in the volumetric rate of fluids passing up the riser. A large portion of this volumetric increase occurs immediately downstream of the feed injection point. Previous feed distributors have allowed this volumetric expansion to occur in a relatively uncontrolled fashion. The uncontrolled volumetric expansion occurring simultaneously with mixing of catalyst and hydrocarbon feed results in mal-distribution that adversely effects the quantity and quality of the products obtained from the cracking reaction. This mal-distribution is caused by turbulent backmixing as well as quiescent zones in the riser section immediately downstream of the feed injection point.
A number of other references also disclose different arrangements for injecting feed radially into a transversely flowing stream of catalyst that is passing through a riser. These methods and apparatus have in common the use of a flow restriction or an orifice arrangement that can provide a venturi effect. Examples of such arrangements are shown in U.S. Pat. No. 5,358,632 issued to Hedrick, U.S. Pat. No. 5,338,438 issued to Demoulin; U.S. Pat. No. 5,205,992 issued to van Ommen et al., U.S. Pat. No. 5,552,119 issued to Holmes and U.S. Pat. No. 5,562,818 issued to Hedrick. These patents all show the radial injection of feed immediately at or downstream of the choke point formed by the restriction or venturi orifice arrangement. These methods and apparatus have the advantage of eliminating, to some degree, turbulence associated with the injection of feed and the rapid expansion of the feed as it contacts the hot catalyst. All of these arrangements, however, suffer from at least one defect associated with their continued use in a commercial process. Some of the apparatus are complicated and difficult to fabricate or install. There is also the problem of maintenance of the apparatus for radially dispersing the flow. High particle velocities create a very erosive environment and is likely to cause rapid erosion of any unprotected metals surfaces and high erosion of surfaces even with abrasion resistant coverings. Therefore, use of such venturi arrangements will require designs that inherently protect the nozzles as well as allowing for easy replacement and maintenance of the apparatus. Finally, there is also the need to provide a system that is flexible and easily modified.